How to have numeric URLs in Posts and Pages

Hope the following code might help

add_filter( 'wp_unique_post_slug', 'mg_unique_post_slug', 10, 6 );

/**
 * Allow numeric slug
 *
 * @param string $slug          The slug returned by wp_unique_post_slug().
 * @param int    $post_ID       The post ID that the slug belongs to.
 * @param string $post_status   The status of post that the slug belongs to.
 * @param string $post_type     The post_type of the post.
 * @param int    $post_parent   Post parent ID.
 * @param string $original_slug The requested slug, may or may not be unique.
 */
function mg_unique_post_slug( $slug, $post_ID, $post_status, $post_type, $post_parent, $original_slug ) {
    global $wpdb;

    // don't change non-numeric values
    if ( ! is_numeric( $original_slug ) || $slug === $original_slug ) {
        return $slug;
    }

    // Was there any conflict or was a suffix added due to the preg_match() call in wp_unique_post_slug() ?
    $post_name_check = $wpdb->get_var( $wpdb->prepare(
        "SELECT post_name FROM $wpdb->posts WHERE post_name = %s AND post_type IN ( %s, 'attachment' ) AND ID != %d AND post_parent = %d LIMIT 1",
        $original_slug, $post_type, $post_ID, $post_parent
    ) );

    // There really is a conflict due to an existing page so keep the modified slug
    if ( $post_name_check ) {
        return $slug;
    }

    // Return our numeric slug
    return $original_slug;
}

The code is taken from http://magnigenie.com/how-to-achieve-numeric-urls-in-wordpress/